It is often necessary or desirable to identify the gas content in a liquid. For example, it may be desirable to measure the carbon dioxide or oxygen content in water, where the measurements are used to help promote plant or animal growth in the water. As a particular example, algae can be used to produce biofuels, and the carbon dioxide content of water in which the algae grows can be measured and controlled. Ideally, carbon dioxide measurements can be used to determine how to treat the water in order to increase or maximize algae growth.
Various types of gas sensors have been developed, but they usually suffer from one or more drawbacks. For example, non-dispersive infrared (NDIR) spectroscopic sensors have been used to measure gas content in air. However, direct measurements of gas content in liquid using an NDIR spectroscopic sensor are difficult. This is due to a number of factors, such as the high attenuation of liquid.
Gas sensors with sensitive polymer-based membranes have also been used to measure the gas content in liquid. However, these sensors can suffer from short-term and long-term drift and may therefore require frequent calibration. Also, the polymer-based membranes can become soiled by contaminants and may require frequent cleaning. In addition, these types of membranes typically have low overall lifetimes, meaning the membranes need frequent replacement.